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The most common use of the colon is to inform the reader that what follows the colon proves, explains, defines, describes, or lists elements of what preceded it. In modern American English usage, a complete sentence precedes a colon, while a list, description, explanation, or definition follows it. The elements which follow the colon may or may not be a complete sentence: since the colon is preceded by a sentence, it is a complete sentence whether what follows the colon is another sentence or not. While it is acceptable to capitalize the first letter after the colon in American English, it is not the case in British English, except where a proper noun immediately follows a colon.[2]

colon used before list

Williams was so hungry he ate everything in the house: chips, cold pizza, pretzels and dip, hot dogs, peanut butter and candy.

colon used before a description

Jane is so desperate that she'll date anyone, even Tom: he's uglier than a squashed toad on the highway, and that's on his good days.

colon before definition

For years while I was reading Shakespeare's Othello and criticism on it, I had to constantly look up the word "egregious" since the villain uses that word: outstandingly bad or shocking.

colon before explanation

I had a rough weekend: I had chest pain and spent all Saturday and Sunday in the emergency room.

Some writers use fragments (incomplete sentences) before a colon for emphasis or stylistic preferences (to show a character's voice in literature), as in this example:

Dinner: chips and juice. What a well-rounded diet I have.

The Bedford Handbook describes several uses of a colon. For example, one can use a colon after an independent clause to direct attention to a list, an appositive or a quotation, and it can be used between independent clauses if the second summarizes or explains the first. In non-literary or non-expository uses, one may use a colon after the salutation in a formal letter, to indicate hours and minutes, to show proportions, between a title and subtitle, and between city and publisher in bibliographic entries.[3]

Luca Serianni, an Italian scholar who helped to define and develop the colon as a punctuation mark, identified four punctuational modes for it: syntactical-deductive, syntactical-descriptive, appositive, and segmental.[4] Although Serianni wrote this guide for the Italian language, his definitions apply also to English and many other languages.

An appositive colon also separates the subtitle of a work from its principal title. Dillon has noted the impact of colons on scholarly articles,[8][9] but the reliability of colons as a predictor of quality or impact has also been challenged.[10][11] In titles, neither needs to be a complete sentence as titles do not represent expository writing:

Like a dash or quotation mark, a segmental colon introduces speech. The segmental function was once a common means of indicating an unmarked quotation on the same line. The following example is from the grammar book The King's English:

Benjamin Franklin proclaimed the virtue of frugality: A penny saved is a penny earned.

This form is still used in written dialogues, such as in a play. The colon indicates that the words following an individual's name are spoken by that individual.

In many European languages, the colon is usually followed by a lower-case letter unless the upper case is required for other reasons, as with British English. German usage requires capitalization of independent clauses following a colon.[13]Dutch further capitalizes the first word of any quotation following a colon, even if it is not a complete sentence on its own.[14]

In print, a thin space was traditionally placed before a colon and a thick space after it. In modern English-language printing, no space is placed before a colon and a single space is placed after it. In French-language typing and printing, the traditional rules are preserved.

One or two spaces may be and have been used after a colon. The older convention (designed to be used by monospaced fonts) was to use two spaces after a colon.[15]

The English word "colon" is from Latincolon (pl.cola), itself from Ancient Greekκῶλον (kôlon), meaning "limb", "member", or "portion". In Greek rhetoric and prosody, the term did not refer to punctuation but to the expression or passage itself. A "colon" was a section of a complete thought or passage. From this usage, in palaeography, a colon is a clause or group of clauses written as a line in a manuscript.[16] In the punctuation system devised by Aristophanes of Byzantium in the 3rd century BC, the end of such a clause was thought to occasion a medium-length breath and was marked by a middot ⟨·⟩. (This was only intermittently used, but eventually revived as the ano teleia, the modern Greeksemicolon.[17]) A double dot symbol ⟨⁚⟩, meanwhile, later came to be used as a full stop or to mark a change of speaker. A variant was introduced to English orthography around 1600, marking a pause intermediate between a comma and a period.[18] As late as the 18th century, the appropriateness of a colon was still being related to the length of the pause taken when reading the text aloud, but silent reading eventually replaced this with other considerations.[19]

A special triangular colon symbol is used in IPA to indicate that the preceding sound is long. Its form is that of two triangles, each a little larger than a point (dot) of a standard colon, pointing toward each other. It is available in Unicode as modifier letter triangular colon, Unicode U+02D0 (ː). A regular colon is often used as a fallback when this character is not available, and in the practical orthography of some languages which have a phonemic long/short distinction in vowels.

If the upper triangle is used without the lower one (modifier letter half triangular colon, Unicode U+02D1: ⟨ˑ⟩), it designates a "half-long" vowel.[23]

The colon is used in mathematics, cartography, model building, and other fields—in this context it denotes a ratio or a scale, as in 3:1 (pronounced "three to one"). When a ratio is reduced to a simpler form, such as 10:15 to 2:3, this may be expressed with a double colon as 10:15::2:3; this would be read "10 is to 15 as 2 is to 3". This form is also used in tests of logic where the question of "Dog is to Puppy as Cat is to _____?" can be expressed as "Dog:Puppy::Cat:_____". Unicode provides a distinct character U+2236∶ratio for mathematical usage. In some languages (e.g. German, Russian and French), the colon is the commonly used sign for division (instead of ÷).

S={x∈R:1<x<3}{\displaystyle S=\{x\in \mathbb {R} :1<x<3\}} (S is the set of all x in R{\displaystyle \mathbb {R} } (the real numbers) such that x is strictly greater than 1 and strictly smaller than 3)

In Microsoft Windowsfilenames, the colon is prohibited. By default, however, filenames use the Segoe UI font, and the tone letter U+A789 is identical to the colon in this font, so it can be used in place of the colon.

Several programming languages use the colon for various purposes.

A number of programming languages, most notably Pascal and Ada, use a colon immediately followed by an equality sign (:=), in which case the colon and the equality sign are considering to compose to an independent assignment sign; this can be represented in Unicode as U+2254≔colon equals.

The colon is also used in many operating systems commands. It is often used as a single post-fix delimiter, signifying a token keyword had immediately preceded it or the transition from one mode of character string interpretation to another related mode. Some applications, such as the widely used MediaWiki, utilize the colon as both a pre-fix and post-fix delimiter.

In wiki markup, the colon is often used to indent text. Common usage includes separating or marking comments in a discussion as replies (see WP:INDENT), or to distinguish certain parts of a text.

Markup

Renders as

Normal text.
:Dented text by the means of a colon.
::The gap increases with colon number.

The ML languages (including Standard ML and OCaml) have the above reversed, where the double colon (::) is used to add an element to the front of a list; and the single colon (:) is used for type guards.

MATLAB uses the colon as a binary operator that generates vectors, as well as to select particular portions of existing matrices.

In Python, which uses indentation to indicate blocks, the colon is used in statements to indicate that the next line is the start of an indented block.

On the Internet, a colon, or multiple colons, is sometimes used to denote an action (similar to how asterisks are used)[original research?] or to emote (for example, in vBulletin). In the action denotation usage it has the inverse function of quotation marks, denoting actions where unmarked text is assumed to be dialogue. For example:

Tom: Pluto is so small; it should not be considered a planet. It is tiny!

^Paterson, Derek (2009-11-19). "How many spaces after a colon?". Absolute Write forums. Post 4. Retrieved 2012-11-04. Back in the typewriter day, when fading ink ribbons could result in commas being mistaken for periods and vice versa, typists were taught to insert 2 spaces after the period to differentiate between the two. The same happened with colons and semicolons: 2 spaces were left after a colon; 1 space after a semicolon.